Symfony is not the biggest change in Drupal 8

The title of this blog post it's based on some of the topics Larry Garfield AKA @Crell talk about at the Acquia podcast. "Drupal 8 Wins: Avoiding the Dead Hook Blues" Part1 , Part2 & Part3

"Adopting various Symfony components has had a huge impact on Drupal, however people should not confuse the parts that are done with Symfony versus the parts that are done with PHP 5.3. PHP 4 and PHP 5.3 are different languages: conceptually, structurally; they happen to be mostly backward-compatible, but that's about it. Drupal 8's biggest shift is that it is a PHP 5.3 system. We have ported Drupal to a new language. That is the biggest jump for people. Symfony is a PHP 5.3 component library that we're using a lot of pieces of to save time and because they have already solved a lot of hard problems in ways that are very good."
~Larry Garfield

Recaping some of the mentioned concepts I decided to curate this list including a summary for some of those concepts and links for more information and reference.

Composer
Composer is a tool for dependency management in PHP. It allows you to declare the dependent libraries your project needs and it will install them in your project for you.https://getcomposer.org/